# Old Skool calls
use Win32::API;
my $getwindir = new Win32::API(kernel32 => GetWindowsDirectoryA => [qw(P N)], "N")
  or die "Can't locate function: $!";
# ... or use 'PN' for the 3rd parameter, if Win32::API is recent enough (version?)
my $buffer = "\0" x 300;
my $length = $getwindir->Call($buffer, length $buffer);
print substr $buffer, 0, $length;